1. Field of the Invention
The present invention generally relates to an infrared laser optical element and a manufacturing method therefor. And more specifically, it relates to an optical coated element such as a focusing lens, window, or the like to be used for an infrared laser applied to machining processes such as a high-power carbon dioxide (CO2) laser, and a manufacturing method therefor.
2. Description of the Related Art
Conventionally, in a processing machine using an infrared laser such as a high-power carbon dioxide laser (emission wavelength: 10.6 xcexcm), zinc selenide (ZnSe), as being transparent to infrared rays, have been used as a substrate of optical elements for the resonator and focusing systems such as an output coupling mirror, rear mirror, focusing lens and the like. On the other hand, mirrors using silicon (Si) or copper (Cu) as the optical substrate have been used for optical elements of the reflection system.
An optical thin film is coated on the surface of the substrate for these optical elements, whereby the optical elements have desired optical performance. Particularly, in case of an optical element for a carbon dioxide laser, strong energy is supplied to the surface and inside of the optical element due to reflection or transmission of laser light. So thorium fluoride (ThF4) or semiconductor materials such as zinc selenide (ZnSe) and zinc sulphide (ZnS) having low film absorption have been used for materials of the optical thin film to be coated on the surface of the substrate.
Recently, as material processing by a high-power laser has been made practicable, highly accurate and stable processing have been needed, and the necessity has rapidly increased for suppressing the absorption of laser beams by an optical thin film to be coated on optical elements. Under such circumstances, research and development of film manufacturing technology has been carrying out in order to obtain optical thin films for lasers having low laser beam absorption and high durability characteristics.
For example, U.S. Pat. No. 6,020,992 discloses a low-absorption of 0.11 through 0.12 coating film for infrared laser optical elements, that was obtained by laminating a barium fluoride (BaF2) film having a low refractive index and a ZnSe film having a high refractive index, alternately, on a ZnSe substrate.
However, in the coating film disclosed in the above mentioned U.S. Patent, the ZnSe film tend to be easily stripped, as the ZnSe film formed on the uppermost layer is thicker than the BaF2 film in addition, since the BaF2 film has a hygroscopic property, in case where the surface of the ZnSe film is damaged, the BaF2 film easily deteriorates through the damaged portion, so that it becomes difficult to keep the absorption of the entire coating film to be low as an optical element.
Furthermore, if the ZnSe film is thick, the transmittance of spectrum becomes extremely sensitive to the thickness of the ZnSe film at the infrared laser wavelength region. Since the transmittance changes substantially due to a slight difference of the film thickness, it becomes necessary to strictly control the film thickness in the film manufacturing process in order to maintain a desired transmittance. However, it is difficult to control the film thickness so that the anti-reflective function can show a the highest level due to optical spectral characteristics of the coating film. Furthermore, the ZnSe film being thick, it requires a lengthy vapor deposition time for manufacturing the film, and so productivity is low.
Furthermore, an example to improve the the mechanical characteristics of optical coatings for a carbon dioxide laser is mentioned in which a BaF2 and a ZnSe films are deposited on a ZnSe substrate with vapor deposition by means of electron beam heating assisted by xenon (Xe) gas ion beams:
pp. 188-193 of xe2x80x9cLaser optical coatings produced by ion assisted depositionxe2x80x9d, Thin Solid Films, 214 (1992) by S. Scaglione et al.
Moreover, another example is introduced in which a BaF2 film is formed on a ZnSe substrate by argon (Ar) gas ion assisted deposition:
pp. 335-348 of xe2x80x9cThe effect of hydrogen concentration in conventional and IAD coatings on the absorption and laser induced damage at 10.6 xcexcmxe2x80x9d on pp. 335-348of SPIE Vol. 1848 Laser-Induced Damage in Optical Materials by M. Rache et al., 1992.
However, in these documents, there is no mention of a construction of an infrared laser optical element whose laser beam absorption has been lowered to not more than 0.15%.
The object of this invention is to provide an infrared laser optical element and a manufacturing method for the same, wherein the laser beam absorption can be lowered to 0.15% at most, and a highly dense and moisture resistant BaF2 film is provided to improve durability.
The infrared laser optical element according to the invention is provided with an optical substrate having a main surface, a BaF2 film formed on said main surface, and a ZnSe film formed on BaF2 film, wherein smoothing treatment is applied on the main surface of the optical substrate.
In the above-mentioned optical element, since the main surface of the optical substrate has been smoothed, a dense BaF2 film can be formed by ion assisted deposition. Therefore, moisture resistance of the BaF2 film is high. Accordingly, even if the thickness of the ZnSe film formed on the BaF2 film is reduced, the optical coating having high durability can be applied on the optical element. As a result, there is minimal deterioration of the BaF2 film even if surface of the uppermost ZnSe layer is damaged, and so the absorption of the entire coating can be maintained to a value as low as 0.15% at most as the optical element.
Furthermore, since the main surface of the optical substrate has been smoothed and so the surface roughness is small, the outer surface of the coated optical element may not be easily stained, nor damaged by laser irradiation in comparison with the case where the surface roughness is large. As a result, an optical element maintaining an excellent appearance can be provided.
The manufacturing method for an infrared laser optical element according to the present invention comprises steps of smoothing the main surface of an optical substrate by irradiating Xe gas ion beams thereon, forming a BaF2 film by Xe gas ion assisted vapor deposition on said smoothed main surface of the substrate, and forming a ZnSe film on said BaF2 film.
By the above-mentioned manufacturing method, a dense BaF2 film can be formed, and therefore, the moisture resistance of the BaF2 film is high, because a BaF2 film is formed by ion beam assisted vapor deposition on the main surface of the substrate which has been smoothed.